The present invention relates generally to using at least one green sheet that is originally very thin with the help of at least one thicker green sheet. An adhesion barrier to build multi-layer ceramic laminates and process thereof is also disclosed. Basically, the present invention relates to a structure and method for forming laminated structures and more particularly to a structure and method for fabricating multi-density, multi-layer ceramic products using at least one very thin green sheet and/or at least one green sheet with very dense electrically conductive patterns on top of at least one thicker green sheet.
Multi-layer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, a MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched in between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have tiny holes or via holes. Prior to lamination, the via holes are filled with an electrically conductive paste, such as, a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few.
Generally, ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After punching, metal paste screening, stacking and laminating, the green sheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. The present invention is directed to the screening, stacking and lamination steps of this process.
In the MLC packaging industry it is very common to use green sheets of various thicknesses. The thicknesses can typically vary from 6 mils to 30 mils and in general the art of punching and metallizing these layers are well known. Green sheet thicknesses below 6 mils, in general, are very scarcely used. This is due to a variety of reasons, such as, for example, handling, screening and stacking of green sheets thinner than 6 mils pose tremendous challenges. In fact the use of one to two mils thick ceramic green sheets, which are punched and screened, using traditional MLC technology does not exist.
Also, in the MLC packaging industry it is very common to use capacitor layers. The capacitance necessary in a package depends on the design and such capacitance is obtained by choosing proper dielectric layer thickness and metal area within a layer. The industry is always striving for higher capacitance and since the metal area is maxing out for a given substrate size it is necessary to use thinner dielectric layers between electrodes to obtain the required capacitance. For example, as a rule of thumb one could double the capacitance for a given dielectric system and electrode metal area by decreasing the dielectric layer thickness by half Additionally the number of layers needed for capacitance in a package as well has been reduced by about 50 percent. The reduction in the number of layers is desirable, as it reduces the cost and the process of making the substrate.
U.S. Pat. Nos. 5,254,191 and 5,474,741 (Mikeska) teaches the use of flexible constraining layers to reduce X-Y shrinkage during firing of green ceramic bodies. But these flexible constraining layers, among other things, do not act as an adhesion barrier during lamination.
However, the present invention forms laminated multi-density, multi-layer ceramic structures using at least one very thin green sheet and/or at least one green sheet with very dense electrically conductive patterns on top of at least one thicker green sheet. An adhesion barrier that is useful during the lamination process is also utilized to build these multi-density, multi-layer ceramic laminates.
Bearing in mind the problems and deficiencies of the prior art it is therefore one purpose of the present invention to provide a novel method and structure for producing metallized thin green sheets, including sub-structures in multi-layer ceramic packages with novel adhesion barrier therein.
Another purpose of this invention is to have a structure and a method that will provide a semiconductor substrate with at least one capacitor layer or with at least one fine line patterned conductive metal layer.
Yet another purpose of this invention is to provide a structure and a method that will ensure multiple thin layers in a multilayer ceramic package.
Still another purpose of the present invention is to provide a structure and method that will ensure higher capacitance in a multi-layer ceramic package.
Yet another purpose of the present invention is to have a structure and a method for fine line pattern using thin green sheets in multi-layer ceramic packages.
Still yet another purpose of the present invention is to provide a structure and a method for metallizing a thin green sheet without any detrimental distortion.
Still another purpose of the present invention is to have a structure and a method that will ensure handling of thin green sheets for multi-layer ceramic packages.
It is another purpose of the invention to have a structure and a method that produces a multilayer ceramic package that is predictable and repeatable.
Another purpose of the present invention is to laminate several stacked green sheets with novel adhesion barrier to produce sub-structures.
Yet another purpose of the present invention is to use at least one adhesion barrier in lamination of multilayer ceramic packages.
Other purposes, objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
Therefore, in one aspect this invention comprises a method of fabricating at least one multi-density semiconductor substrate comprising the steps of:
(a) forming at least one electrically conductive feature on at least one thick green sheet;
(b) providing at least one thin green sheet with at least one via hole;
(c) aligning and placing said thin green sheet over said thick green sheet, such that at least a portion of said electrically conductive feature is in contact with at least a portion of said via hole;
(d) dusting lamination surfaces with at least one inorganic adhesion barrier powder;
(e) tacking and bonding said at least one thin green sheet to said at least one thick green sheet, and thereby fabricating said multi-density semiconductor substrate.
In another aspect this invention comprises a multi-density substrate comprising at least one thin green sheet with at least one via hole in intimate contact with at least one thick green sheet, wherein at least one electrically conductive feature is sandwiched between said thin green sheet and said thick green sheet, and wherein at least a portion of said via hole is in contact with at least a portion of said at least one electrically conductive feature, and thereby forming said multi-density substrate.
In yet another aspect this invention comprises a multi-density substrate comprising at least one thin green sheet with at least one via hole in intimate contact with at least one thick green sheet having at least one electrically conductive material in secure contact with said thin green sheet, and wherein at least a portion of said via hole is in contact with at least a portion of said electrically conductive material, and wherein at least a portion of at least one surface of said thick and/or thin green sheet has at least one layer of at least one adhesive barrier material, and thereby forming said multi-density substrate.